The present invention relates to the field of recording devices generally known as ink jet printers. More particularly, in one embodiment, the invention provides a recording head comprising an electret transducer effective to eject a quantity of ink to a recording medium.
Variations of direct marking electronic ink jet-type printers and their components have been well known for some number of years. Ink-jet printers may be classified in two general categories; continuous ink flow and drop-on-demand printers. In the continuous ink jet system, fine droplets of ink are continuously ejected from the printhead. Among the continuous fine droplets so ejected, those droplets required to effect the recording are selectively deflected and deposited to a recording surface.
The continuous ink jet systems generate a continuous stream of ink "drops" on the order of about a million drops per second. The drops are electrically charged and selectively deflected onto the recording medium or to a gutter-waste ink collection system. The continuous ink jet-type printer has met with substantial success, and there are several commercial systems operating on the continuous ink jet principle. However, there are many drawbacks to the continuous ink jet system. One shortcoming relates to the limited speed of the print system since relatively few such ink jet streams are typically used.
Drop-on-demand type ink jet printers are advantageous in some respects compared to the continuous type due primarily to a reduction in complexity. Drop-on-demand type systems do not require components such as ink charge inducers and a deflection controlling mechanism for separating the continuous ink droplets and collecting and recycling those not selected for printing. The drop-on-demand type system is therefore somewhat simpler in structure and may be minimized in size. The drop-on-demand type ink jet printers are designed to controllably eject an ink drop only as required. In a drop-on-demand printhead, multiple ink nozzles may be arranged in an array and thereby improve the speed and performance of the drop-on-demand type printers relative to continuous type ink jet printers. An early drop-on-demand device is described in U.S. Pat. No. 2,512,743 issued Jun. 27, 1950 to C. W. Hansell. Later devices include those disclosed in U.S. Pat. No. 3,747,120 issued Jul. 17, 1973 to Stemme. Further discussion of the drop-on-demand type system may be found in IEEE Transactions on Industry Applications, Vol. IA-13, No. 1, January/February 1977.
The above described early drop-on-demand type ink jet type devices remain complex, utilizing energy producing elements of bimorph or monomorph ceramic piezoelectric material such as PZT (lead zirconium titanate). The piezo type printers remained mechanically complex and difficult to manufacture. The relatively large size of the piezo transducer prevents a close spacing of the ink ejecting nozzles and physical limitations inherent with the piezo transducer result in a low ink drop velocity. Also, the piezo vibrating element is technically difficult to manufacture and assemble. The piezo based devices are generally limited to between 10 and 60 nozzles. Further, the piezo-type devices require a voltage in the range of from about 100 to 200 volts depending on the piezo material employed. Overall, such limitations combine to result in a relatively low print speed, even when a moving shuttle type print head is employed.
Somewhat more recently, thermal based ink jet print systems have been described, for example, in U.S. Pat. No. 4,296,421 issued Oct. 20, 1981 to Hara et al., and U.S. Pat. No. 4,680,859 issued Jul. 21, 1987 to Samuel A. Johnson. The nozzles in a thermal ink jet system may be arranged in a very close pattern, with about 50 to 70 nozzles per printhead possible using semiconductor based manufacturing technologies. Indeed, even full page wide printers comprising from about 2400 to 4800 nozzles in line at a density of about 400 spots per inch have been manufactured, and some have print speeds of up to 100 pages per minute.
Even having met with considerable commercial success, thermal ink jet systems still have many shortcomings and limitations. For example, since thermal energy is used for ink drop generation, the ink solution must be superheated to several hundred degrees Fahrenheit, in order to generate the vapor bubble causing ejection of a drop of aqueous based ink. Ink additives which may greatly improve the print quality are excluded from the thermal ink jet systems as they are detrimental to the reliability of the heating elements.
Further associated with thermal based systems, repeated heating and associated collapsing pressure serves to limit the useful life of thermal print heads. Another problem relates to the very low overall thermal drop generation mechanism efficiency, which is on the order of about 0.005%. The thermal systems generate considerable excess heat which causes severe thermal effect problems, particularly in the larger printers. Still yet another problem encountered in the thermal ink jet printing system is that of water absorption. Due to the qualities of aqueous ink, absorption onto the recording medium often results in a cockle or paper wrinkling. This unfortunate result prevents good registration of ink onto the paper and detrimentally affects print and color quality.
From the above it is seen that an improved ink jet print head device and associated method of fabrication is desired not only to provide print head and associated printers with improved performance but also to provide devices which may be simpler to manufacture and use and which are therefore more reliable.